The present invention relates to an interactive cable television system of the type which effects multiple utilization of the transmission paths in a total service area which is subdivided into service regions and includes a central distributing/collecting/receiving/transmitting/exchanging office, and a supply network formed in part of parallel cables in a tree structure and, in the network regions adjacent one or more users, in a star structure.
In addition to the conventional purpose of distributing radio and television programs, it is desirable to provide the capability for conducting individual broadband information and narrow band data exchange between users and the cable television central office. Depending on whether such a system is to be a new installation or an expansion of existing devices together with a modification of existing distribution system networks and devices, various different design concepts may be employed. Additionally, the number and type of distribution and individual services to be made available as well as the number and extent of data exchange participation of the users are of decisive significance to the design selection.
The publication "Rundfunktechnische Mitteilungen" (Radio Technical Information), Volume 20, No. 5 (1976), at pages 173-182, discloses possibilities and problems associated with technical design for a two-way cable television system. Suitable services to be performed, i.e.: unlimited information distribution available to all users; limited information distribution to selected users; information collecting; and dialog with the central office, require, for economical handling, appropriate network design for the transmission channels in the distribution direction and for the return path back to the central office, as well as a suitable concept for a transmission system in which a distributing and collecting system for television signals, digital transmission systems, transmission procedures, and terminal equipment at the user's end and devices in the central office, as well as still picture transmission systems, are included.
Regarding network design, the above-cited publication discloses, for example, structures which start out with a tree-shaped network, which is most favorable for a purely distributing system. An increase in the transmission capacity required, in particular, for individual services, according to this reference, will be handled, for example, by additional parallel cables in the tree network, or, if required, by the same cables with which parts of the tree network are connected with the central office in the form of a star or via loop lines. These approaches lead to a tree-star structure, a star-tree structure or a loop-tree structure at the active network regions or planes where amplifier locations are provided. In the passive network regions or planes near the users, which require considerable expenditures for lines, two alternatives are emphasized in that reference:
If the expenditures are transferred into the transmission systems, the tree structure can be retained. Multiplex formation in frequency- or time-multiplex form leads to a large number of individual data and still picture channels;
From a star point a cable leads to a respective user. A star point is the point of intersection with the higher-order tree network. The bandwidth for transmission on such a cable as well as in the higher-order network is to be divided for separating upstream and downstream channels.
Compared to the tree network, the star network offers greater security against listening-in and interference in the return channel as well as against unauthorized watching of pay programs. A special code, i.e. scrambling, is not required for this purpose.